I have seen quite a few popular pagans convert back to christianity over the years. What's strange is that it's not the christians who make the biggest deal of it, it's us. The community treats it like a betrayal, or that we were hoodwinked and they were faking it the whole time or something. Here is where we have to be different than other religious groups. We can't pressure or shame people to convert into paganism, and we shouldn't pressure or shame people who de-convert either. "Do what thou wilt" means each spiritual journey is valid, even if it leads back to christianity. Even if they immediately turn around and start calling us sinners or whatever, the most powerful thing we can do is ignore them. Making a big deal out of it makes it into a big deal. People change, it is the way of things. Bid them well when they leave, let it happen without fanfare, and don't take it personally.